1. Technical Field
The present disclosure relates generally to measurement of sticky molecules, and more specifically to techniques for preventing interaction of sticky molecules with interfaces inside of an instrument.
2. Background Information
Measurement of Nitric acid (HNO3), ammonia (NH3) and other polar molecules has proven increasingly important in the understanding of atmospheric processes. Their large dipole moments and hydrophilic properties are primary factors in their nucleation and condensation roles in atmospheric aerosol formation. For example, nitric acid is an important photochemical product during NOx oxidation and is an important test of our understanding of modeled photoreactivity in the atmosphere. However, the large dipoles and hydrophilic properties of these polar molecules render them difficult to detect on timescales less than approximately 1 minute. These polar molecules may not only be “sticky” upon interfaces (i.e. surfaces forming a common boundary among two different phases) inside of an instrument (e.g., an infrared absorption spectrometer) by virtue of their own interactions with polar surface groups, but may also be attracted to any points where adsorbed or liquid water accumulates. In light of their properties, HNO3 and NH3 may be considered “sticky molecules”. As used herein, the term “sticky molecule” refers to the class of polar molecules having large dipole moments and hydrophilic properties that cause them to temporarily bind to interfaces. In addition to HNO3 and NH3, formaldehyde (CH2O), acetone (C3H6O), as well as other molecules that act as a Lewis acid or a Lewis base may be considered sticky molecules.
The consequence of the interactions of sticky molecules with interfaces inside of instruments may be significantly lengthened response time of an output signal to rapid changes in concentration, which in some cases may increase detection times to 10-100 seconds(s). For reference, measurements of non-sticky molecules, such as CH4, CO2 and N2O, are typically limited by the sampling flow rate through the instrument and the associated detection volume, and can therefore often achieve detection times less than 0.2 s.
Previous attempts to address the issue of lengthened response time and reduced detection bandwidth have included using materials with non-polar chemical groups, or chemically coating surfaces with permanent non-polar surfactants, which may reduce aggregation of both water and other dipolar species. The surface materials, for example PolyTetraFluoroEthylene (PTFE) or PerFluoroAlkoxy (PFA), or the coatings, for example fluorinated alkylsiloxanes, are generally applied during instrument configuration or manufacture. Such techniques may, at least initially, improve response time. However, the effects typically diminish over the course of instrument usage. An interface that is initially pristine is compromised when as little as a single monolayer of salt or inorganic matter coats the interfaces. Thus, the response time of the instrument may gradually become slower, as an increasing fraction of the interfaces becomes coated in adsorptive matter. One may attempt to clean the interfaces with solvents, to try to restore some of the initial benefits. However, recoating or cleaning or recoating generally involves discontinuing use of the instrument, bringing any internal vacuum to atmospheric pressure, and potentially disassembling significant portions of the instrument. Considerable instrument downtime may be incurred.
Further, coating interfaces with PTFE or PFA often leads to response times that are highly dependent upon relative humidity. The presence of water at the interfaces may be a primary factor behind the “stickiness”. Attempts have been made to lightly heat portions of instruments, to attempt to remove water. However, such an approach may not successfully remove all water because of a) the possibly strong binding between water and the surface, which could be larger than the water-water interaction, and b) the presence of small crevices and pockets in all but the most pristine surfaces, which could be difficult to evacuate with heat. It would be desirable to find a technique that was not so highly dependent on humidity and trapped water.
Accordingly, there is a need for improved techniques for preventing interaction of sticky molecules with interfaces inside of an instrument.